triangulation edge appears black in render

[tinuz]

Hi,

I have a scene with 2 "stepped" planes,(as in modeled as a harmonica)
with a vraymat with a bit of transparency, some colored sphere lights

the planes are deformed by a lattice
when i render , black lines appear exactly on the places where the quad faces are being triangulated during rendering:

is this a bug?
maya 2012, osx 10.6.7, vray 20003